4 use super::{FusedIterator, TrustedLen};
6 /// An iterator that repeats an element endlessly.
8 /// This `struct` is created by the [`repeat`] function. See its documentation for more.
10 /// [`repeat`]: fn.repeat.html
11 #[derive(Clone, Debug)]
12 #[stable(feature = "rust1", since = "1.0.0")]
13 pub struct Repeat<A> {
17 #[stable(feature = "rust1", since = "1.0.0")]
18 impl<A: Clone> Iterator for Repeat<A> {
22 fn next(&mut self) -> Option<A> {
23 Some(self.element.clone())
26 fn size_hint(&self) -> (usize, Option<usize>) {
31 #[stable(feature = "rust1", since = "1.0.0")]
32 impl<A: Clone> DoubleEndedIterator for Repeat<A> {
34 fn next_back(&mut self) -> Option<A> {
35 Some(self.element.clone())
39 #[stable(feature = "fused", since = "1.26.0")]
40 impl<A: Clone> FusedIterator for Repeat<A> {}
42 #[unstable(feature = "trusted_len", issue = "37572")]
43 unsafe impl<A: Clone> TrustedLen for Repeat<A> {}
45 /// Creates a new iterator that endlessly repeats a single element.
47 /// The `repeat()` function repeats a single value over and over again.
49 /// Infinite iterators like `repeat()` are often used with adapters like
50 /// [`take`], in order to make them finite.
52 /// [`take`]: trait.Iterator.html#method.take
54 /// If the element type of the iterator you need does not implement `Clone`,
55 /// or if you do not want to keep the repeated element in memory, you can
56 /// instead use the [`repeat_with`] function.
58 /// [`repeat_with`]: fn.repeat_with.html
67 /// // the number four 4ever:
68 /// let mut fours = iter::repeat(4);
70 /// assert_eq!(Some(4), fours.next());
71 /// assert_eq!(Some(4), fours.next());
72 /// assert_eq!(Some(4), fours.next());
73 /// assert_eq!(Some(4), fours.next());
74 /// assert_eq!(Some(4), fours.next());
76 /// // yup, still four
77 /// assert_eq!(Some(4), fours.next());
80 /// Going finite with [`take`]:
85 /// // that last example was too many fours. Let's only have four fours.
86 /// let mut four_fours = iter::repeat(4).take(4);
88 /// assert_eq!(Some(4), four_fours.next());
89 /// assert_eq!(Some(4), four_fours.next());
90 /// assert_eq!(Some(4), four_fours.next());
91 /// assert_eq!(Some(4), four_fours.next());
93 /// // ... and now we're done
94 /// assert_eq!(None, four_fours.next());
97 #[stable(feature = "rust1", since = "1.0.0")]
98 pub fn repeat<T: Clone>(elt: T) -> Repeat<T> {
99 Repeat { element: elt }
102 /// An iterator that repeats elements of type `A` endlessly by
103 /// applying the provided closure `F: FnMut() -> A`.
105 /// This `struct` is created by the [`repeat_with`] function.
106 /// See its documentation for more.
108 /// [`repeat_with`]: fn.repeat_with.html
109 #[derive(Copy, Clone, Debug)]
110 #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
111 pub struct RepeatWith<F> {
115 #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
116 impl<A, F: FnMut() -> A> Iterator for RepeatWith<F> {
120 fn next(&mut self) -> Option<A> {
121 Some((self.repeater)())
125 fn size_hint(&self) -> (usize, Option<usize>) {
130 #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
131 impl<A, F: FnMut() -> A> FusedIterator for RepeatWith<F> {}
133 #[unstable(feature = "trusted_len", issue = "37572")]
134 unsafe impl<A, F: FnMut() -> A> TrustedLen for RepeatWith<F> {}
136 /// Creates a new iterator that repeats elements of type `A` endlessly by
137 /// applying the provided closure, the repeater, `F: FnMut() -> A`.
139 /// The `repeat_with()` function calls the repeater over and over again.
141 /// Infinite iterators like `repeat_with()` are often used with adapters like
142 /// [`take`], in order to make them finite.
144 /// [`take`]: trait.Iterator.html#method.take
146 /// If the element type of the iterator you need implements `Clone`, and
147 /// it is OK to keep the source element in memory, you should instead use
148 /// the [`repeat`] function.
150 /// [`repeat`]: fn.repeat.html
152 /// An iterator produced by `repeat_with()` is not a `DoubleEndedIterator`.
153 /// If you need `repeat_with()` to return a `DoubleEndedIterator`,
154 /// please open a GitHub issue explaining your use case.
163 /// // let's assume we have some value of a type that is not `Clone`
164 /// // or which don't want to have in memory just yet because it is expensive:
165 /// #[derive(PartialEq, Debug)]
166 /// struct Expensive;
168 /// // a particular value forever:
169 /// let mut things = iter::repeat_with(|| Expensive);
171 /// assert_eq!(Some(Expensive), things.next());
172 /// assert_eq!(Some(Expensive), things.next());
173 /// assert_eq!(Some(Expensive), things.next());
174 /// assert_eq!(Some(Expensive), things.next());
175 /// assert_eq!(Some(Expensive), things.next());
178 /// Using mutation and going finite:
183 /// // From the zeroth to the third power of two:
184 /// let mut curr = 1;
185 /// let mut pow2 = iter::repeat_with(|| { let tmp = curr; curr *= 2; tmp })
188 /// assert_eq!(Some(1), pow2.next());
189 /// assert_eq!(Some(2), pow2.next());
190 /// assert_eq!(Some(4), pow2.next());
191 /// assert_eq!(Some(8), pow2.next());
193 /// // ... and now we're done
194 /// assert_eq!(None, pow2.next());
197 #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
198 pub fn repeat_with<A, F: FnMut() -> A>(repeater: F) -> RepeatWith<F> {
199 RepeatWith { repeater }
202 /// An iterator that yields nothing.
204 /// This `struct` is created by the [`empty`] function. See its documentation for more.
206 /// [`empty`]: fn.empty.html
207 #[stable(feature = "iter_empty", since = "1.2.0")]
208 pub struct Empty<T>(marker::PhantomData<T>);
210 #[stable(feature = "iter_empty_send_sync", since = "1.42.0")]
211 unsafe impl<T> Send for Empty<T> {}
212 #[stable(feature = "iter_empty_send_sync", since = "1.42.0")]
213 unsafe impl<T> Sync for Empty<T> {}
215 #[stable(feature = "core_impl_debug", since = "1.9.0")]
216 impl<T> fmt::Debug for Empty<T> {
217 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
222 #[stable(feature = "iter_empty", since = "1.2.0")]
223 impl<T> Iterator for Empty<T> {
226 fn next(&mut self) -> Option<T> {
230 fn size_hint(&self) -> (usize, Option<usize>) {
235 #[stable(feature = "iter_empty", since = "1.2.0")]
236 impl<T> DoubleEndedIterator for Empty<T> {
237 fn next_back(&mut self) -> Option<T> {
242 #[stable(feature = "iter_empty", since = "1.2.0")]
243 impl<T> ExactSizeIterator for Empty<T> {
244 fn len(&self) -> usize {
249 #[unstable(feature = "trusted_len", issue = "37572")]
250 unsafe impl<T> TrustedLen for Empty<T> {}
252 #[stable(feature = "fused", since = "1.26.0")]
253 impl<T> FusedIterator for Empty<T> {}
255 // not #[derive] because that adds a Clone bound on T,
256 // which isn't necessary.
257 #[stable(feature = "iter_empty", since = "1.2.0")]
258 impl<T> Clone for Empty<T> {
259 fn clone(&self) -> Empty<T> {
260 Empty(marker::PhantomData)
264 // not #[derive] because that adds a Default bound on T,
265 // which isn't necessary.
266 #[stable(feature = "iter_empty", since = "1.2.0")]
267 impl<T> Default for Empty<T> {
268 fn default() -> Empty<T> {
269 Empty(marker::PhantomData)
273 /// Creates an iterator that yields nothing.
282 /// // this could have been an iterator over i32, but alas, it's just not.
283 /// let mut nope = iter::empty::<i32>();
285 /// assert_eq!(None, nope.next());
287 #[stable(feature = "iter_empty", since = "1.2.0")]
288 #[rustc_const_stable(feature = "const_iter_empty", since = "1.32.0")]
289 pub const fn empty<T>() -> Empty<T> {
290 Empty(marker::PhantomData)
293 /// An iterator that yields an element exactly once.
295 /// This `struct` is created by the [`once`] function. See its documentation for more.
297 /// [`once`]: fn.once.html
298 #[derive(Clone, Debug)]
299 #[stable(feature = "iter_once", since = "1.2.0")]
301 inner: crate::option::IntoIter<T>,
304 #[stable(feature = "iter_once", since = "1.2.0")]
305 impl<T> Iterator for Once<T> {
308 fn next(&mut self) -> Option<T> {
312 fn size_hint(&self) -> (usize, Option<usize>) {
313 self.inner.size_hint()
317 #[stable(feature = "iter_once", since = "1.2.0")]
318 impl<T> DoubleEndedIterator for Once<T> {
319 fn next_back(&mut self) -> Option<T> {
320 self.inner.next_back()
324 #[stable(feature = "iter_once", since = "1.2.0")]
325 impl<T> ExactSizeIterator for Once<T> {
326 fn len(&self) -> usize {
331 #[unstable(feature = "trusted_len", issue = "37572")]
332 unsafe impl<T> TrustedLen for Once<T> {}
334 #[stable(feature = "fused", since = "1.26.0")]
335 impl<T> FusedIterator for Once<T> {}
337 /// Creates an iterator that yields an element exactly once.
339 /// This is commonly used to adapt a single value into a [`chain`] of other
340 /// kinds of iteration. Maybe you have an iterator that covers almost
341 /// everything, but you need an extra special case. Maybe you have a function
342 /// which works on iterators, but you only need to process one value.
344 /// [`chain`]: trait.Iterator.html#method.chain
353 /// // one is the loneliest number
354 /// let mut one = iter::once(1);
356 /// assert_eq!(Some(1), one.next());
358 /// // just one, that's all we get
359 /// assert_eq!(None, one.next());
362 /// Chaining together with another iterator. Let's say that we want to iterate
363 /// over each file of the `.foo` directory, but also a configuration file,
369 /// use std::path::PathBuf;
371 /// let dirs = fs::read_dir(".foo").unwrap();
373 /// // we need to convert from an iterator of DirEntry-s to an iterator of
374 /// // PathBufs, so we use map
375 /// let dirs = dirs.map(|file| file.unwrap().path());
377 /// // now, our iterator just for our config file
378 /// let config = iter::once(PathBuf::from(".foorc"));
380 /// // chain the two iterators together into one big iterator
381 /// let files = dirs.chain(config);
383 /// // this will give us all of the files in .foo as well as .foorc
385 /// println!("{:?}", f);
388 #[stable(feature = "iter_once", since = "1.2.0")]
389 pub fn once<T>(value: T) -> Once<T> {
390 Once { inner: Some(value).into_iter() }
393 /// An iterator that yields a single element of type `A` by
394 /// applying the provided closure `F: FnOnce() -> A`.
396 /// This `struct` is created by the [`once_with`] function.
397 /// See its documentation for more.
399 /// [`once_with`]: fn.once_with.html
400 #[derive(Clone, Debug)]
401 #[stable(feature = "iter_once_with", since = "1.43.0")]
402 pub struct OnceWith<F> {
406 #[stable(feature = "iter_once_with", since = "1.43.0")]
407 impl<A, F: FnOnce() -> A> Iterator for OnceWith<F> {
411 fn next(&mut self) -> Option<A> {
412 let f = self.gen.take()?;
417 fn size_hint(&self) -> (usize, Option<usize>) {
418 self.gen.iter().size_hint()
422 #[stable(feature = "iter_once_with", since = "1.43.0")]
423 impl<A, F: FnOnce() -> A> DoubleEndedIterator for OnceWith<F> {
424 fn next_back(&mut self) -> Option<A> {
429 #[stable(feature = "iter_once_with", since = "1.43.0")]
430 impl<A, F: FnOnce() -> A> ExactSizeIterator for OnceWith<F> {
431 fn len(&self) -> usize {
432 self.gen.iter().len()
436 #[stable(feature = "iter_once_with", since = "1.43.0")]
437 impl<A, F: FnOnce() -> A> FusedIterator for OnceWith<F> {}
439 #[stable(feature = "iter_once_with", since = "1.43.0")]
440 unsafe impl<A, F: FnOnce() -> A> TrustedLen for OnceWith<F> {}
442 /// Creates an iterator that lazily generates a value exactly once by invoking
443 /// the provided closure.
445 /// This is commonly used to adapt a single value generator into a [`chain`] of
446 /// other kinds of iteration. Maybe you have an iterator that covers almost
447 /// everything, but you need an extra special case. Maybe you have a function
448 /// which works on iterators, but you only need to process one value.
450 /// Unlike [`once`], this function will lazily generate the value on request.
452 /// [`once`]: fn.once.html
453 /// [`chain`]: trait.Iterator.html#method.chain
462 /// // one is the loneliest number
463 /// let mut one = iter::once_with(|| 1);
465 /// assert_eq!(Some(1), one.next());
467 /// // just one, that's all we get
468 /// assert_eq!(None, one.next());
471 /// Chaining together with another iterator. Let's say that we want to iterate
472 /// over each file of the `.foo` directory, but also a configuration file,
478 /// use std::path::PathBuf;
480 /// let dirs = fs::read_dir(".foo").unwrap();
482 /// // we need to convert from an iterator of DirEntry-s to an iterator of
483 /// // PathBufs, so we use map
484 /// let dirs = dirs.map(|file| file.unwrap().path());
486 /// // now, our iterator just for our config file
487 /// let config = iter::once_with(|| PathBuf::from(".foorc"));
489 /// // chain the two iterators together into one big iterator
490 /// let files = dirs.chain(config);
492 /// // this will give us all of the files in .foo as well as .foorc
494 /// println!("{:?}", f);
498 #[stable(feature = "iter_once_with", since = "1.43.0")]
499 pub fn once_with<A, F: FnOnce() -> A>(gen: F) -> OnceWith<F> {
500 OnceWith { gen: Some(gen) }
503 /// Creates a new iterator where each iteration calls the provided closure
504 /// `F: FnMut() -> Option<T>`.
506 /// This allows creating a custom iterator with any behavior
507 /// without using the more verbose syntax of creating a dedicated type
508 /// and implementing the `Iterator` trait for it.
510 /// Note that the `FromFn` iterator doesn’t make assumptions about the behavior of the closure,
511 /// and therefore conservatively does not implement [`FusedIterator`],
512 /// or override [`Iterator::size_hint`] from its default `(0, None)`.
514 /// [`FusedIterator`]: trait.FusedIterator.html
515 /// [`Iterator::size_hint`]: trait.Iterator.html#method.size_hint
517 /// The closure can use captures and its environment to track state across iterations. Depending on
518 /// how the iterator is used, this may require specifying the `move` keyword on the closure.
522 /// Let’s re-implement the counter iterator from [module-level documentation]:
524 /// [module-level documentation]: index.html
527 /// let mut count = 0;
528 /// let counter = std::iter::from_fn(move || {
529 /// // Increment our count. This is why we started at zero.
532 /// // Check to see if we've finished counting or not.
539 /// assert_eq!(counter.collect::<Vec<_>>(), &[1, 2, 3, 4, 5]);
542 #[stable(feature = "iter_from_fn", since = "1.34.0")]
543 pub fn from_fn<T, F>(f: F) -> FromFn<F>
545 F: FnMut() -> Option<T>,
550 /// An iterator where each iteration calls the provided closure `F: FnMut() -> Option<T>`.
552 /// This `struct` is created by the [`iter::from_fn`] function.
553 /// See its documentation for more.
555 /// [`iter::from_fn`]: fn.from_fn.html
557 #[stable(feature = "iter_from_fn", since = "1.34.0")]
558 pub struct FromFn<F>(F);
560 #[stable(feature = "iter_from_fn", since = "1.34.0")]
561 impl<T, F> Iterator for FromFn<F>
563 F: FnMut() -> Option<T>,
568 fn next(&mut self) -> Option<Self::Item> {
573 #[stable(feature = "iter_from_fn", since = "1.34.0")]
574 impl<F> fmt::Debug for FromFn<F> {
575 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
576 f.debug_struct("FromFn").finish()
580 /// Creates a new iterator where each successive item is computed based on the preceding one.
582 /// The iterator starts with the given first item (if any)
583 /// and calls the given `FnMut(&T) -> Option<T>` closure to compute each item’s successor.
586 /// use std::iter::successors;
588 /// let powers_of_10 = successors(Some(1_u16), |n| n.checked_mul(10));
589 /// assert_eq!(powers_of_10.collect::<Vec<_>>(), &[1, 10, 100, 1_000, 10_000]);
591 #[stable(feature = "iter_successors", since = "1.34.0")]
592 pub fn successors<T, F>(first: Option<T>, succ: F) -> Successors<T, F>
594 F: FnMut(&T) -> Option<T>,
596 // If this function returned `impl Iterator<Item=T>`
597 // it could be based on `unfold` and not need a dedicated type.
598 // However having a named `Successors<T, F>` type allows it to be `Clone` when `T` and `F` are.
599 Successors { next: first, succ }
602 /// An new iterator where each successive item is computed based on the preceding one.
604 /// This `struct` is created by the [`successors`] function.
605 /// See its documentation for more.
607 /// [`successors`]: fn.successors.html
609 #[stable(feature = "iter_successors", since = "1.34.0")]
610 pub struct Successors<T, F> {
615 #[stable(feature = "iter_successors", since = "1.34.0")]
616 impl<T, F> Iterator for Successors<T, F>
618 F: FnMut(&T) -> Option<T>,
623 fn next(&mut self) -> Option<Self::Item> {
624 let item = self.next.take()?;
625 self.next = (self.succ)(&item);
630 fn size_hint(&self) -> (usize, Option<usize>) {
631 if self.next.is_some() { (1, None) } else { (0, Some(0)) }
635 #[stable(feature = "iter_successors", since = "1.34.0")]
636 impl<T, F> FusedIterator for Successors<T, F> where F: FnMut(&T) -> Option<T> {}
638 #[stable(feature = "iter_successors", since = "1.34.0")]
639 impl<T: fmt::Debug, F> fmt::Debug for Successors<T, F> {
640 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
641 f.debug_struct("Successors").field("next", &self.next).finish()